Modular regenerative heat exchanger system

ABSTRACT

A plurality of independently operable regenerative heat exchanger modules ( 1 - 5 ) are provided to regeneratively transfer heat from a hot gas to a cold gas. The regenerative heat exchanger modules are connected to a regenerative heat exchanger system controller ( 1   p - 5   p ) which staggers the operation of each regenerative heat exchanger module to simulate the operation of a rotary regenerative heat exchanger. The regenerative heat exchanger system controller can manually or automatically take selected ones of the regenerative heat exchanger modules off-line while the remaining regenerative heat exchanger modules continue to simulate the operation of the rotary regenerative heat exchanger. Also disclosed are a control system and a method for operating a number of independently operable regenerative heat exchanger modules to simulate the operation of a rotary regenerative heat exchanger.

CLAIM OF PRIORITY

This application claims priority from U.S. Provisional Application No.60/352,097 filed Jan. 23, 2002.

FIELD AND BACKGROUND OF THE INVENTION

The present invention is directed to an apparatus and method fortransferring heat from a hot fluid to a cold fluid using regenerativeheat-transfer techniques.

Regenerative heat-exchangers have long been used as a means fortransferring heat from a hot fluid to a cold fluid. The theory andoperation of regenerative heat-exchangers is well-known and documentedin the prior art.

For example, U.S. Pat. No. 3,225,819 to Stevens discloses a regenerativeheat exchanger having separate regenerative heat exchanger chambers. Anelectro-mechanical control system controls the flow of the hot and coldgases through each of the chambers to provide continuous flow of the hotand cold gases through the system. However, such a regenerative heatexchanger is expensive to fabricate, takes excessive plant space, and isnot very flexible with respect to operation under widely varyingoperating condition. Therefore, it has been supplanted by rotaryregenerative heat exchangers (more commonly called “Ljungstrom wheels”)which are widely used in power plant applications to increase thethermal efficiency of boilers. These rotary regenerative heat exchangersare commercially available from manufacturers such as Air Preheater Co.(USA), Howden Colo. (UK) and others.

However, these rotary regenerative heat exchangers suffer from a numberof disadvantages. For example, they rely on sliding seals to separatethe hot gas from the cold gas. As well documented in the prior art, suchas in U.S. Pat. No. 6,227,150 to Finnemore, the sliding seals getrapidly worn out and result in excessive cross-leakage between the hotgas and the cold gas. The seals therefore require frequent replacementresulting in excessive boiler down-time and excessive maintenance costs.The cross-leakage results also in excessive parasitic power consumptionand reduced boiler fuel-conversion efficiency. Alternately, special sealadjusting devices have to be used to minimize the cross-leakage duringoperation. This greatly adds to the cost and complexity of the rotaryregenerative heat exchanger.

Furthermore, for economy of manufacturing, rotary regenerative heatexchangers are normally manufactured to handle large quantities of gas.The large size causes severe thermal deformation problems during theoperation of the rotary regenerative heat exchanger. Since only two orthree rotary regenerative heat exchangers are generally used in atypical power-plant, a minor breakdown of a component in the rotaryregenerative heat exchanger requires that the boiler be shut down or runat a 50% capacity while repairs are made to the rotary regenerative heatexchanger. This causes large losses in production of electrical energyand loss of revenue to the power plant.

Further, the large size of the rotary regenerative heat exchangergenerally requires a great deal of on-site field fabrication andinstallation, which results in high installation costs.

Yet further, during off-peak hours, when the megawatt-load demand on theboiler is low, the amount of gas flowing through the rotary regenerativeheat exchanger is greatly reduced. The reduced flow results in reducedgas velocity through the rotary regenerative heat exchanger. This, inturn, results in increased deposition of fly-ash on the surface of theheat-sink material of the rotary regenerative heat exchanger. Theoverall result is a loss of heat-transfer efficiency and a need for morefrequent cleaning of the rotary regenerative heat exchanger. Theadditional cleaning using soot-blowers and/or other means generallyincreases the parasitic steam consumption of the regenerative heatexchanger while reducing the overall fuel-conversion efficiency of theboiler.

Therefore, it will be apparent that a need exists for a regenerativeheat exchanger that provides a low cross-leakage and a capability forhandling variable quantities of gases. The regenerative heat exchangershould also provide generally optimum operation throughout the fullturndown range of flow of the gases. Further, the regenerative heatexchanger should not require sliding seals to separate the cold and hotgases thereby reducing process down-time, maintenance costs, parasiticpower and steam consumption. Yet further, the regenerative heatexchanger should use relatively inexpensive, non-thermally-deformable,corrosion-resisting heat-sink material. Such a regenerative heatexchanger should be economical to manufacture, transport, and install atthe job-site. A process using a regenerative heat exchanger with theabove advantages will operate at a higher efficiency with reducedcapital and operating costs.

SUMMARY OF THE INVENTION

In one aspect of the invention, hot and cold gas is alternately passedthrough a plurality of independently operable stationary regenerativeheat exchanger modules to simulate the operation of a rotaryregenerative heat exchanger having revolving heat transfer sectors. Inthe regenerative heat exchanger module, the heat-sink media gets heatedby absorbing the heat from the hot gas, thereby cooling the hot gas.After a period of time, the flow of hot gas to the regenerative heatexchanger module is shut off and cold gas is introduced into theregenerative heat exchanger module. The cold gas gets heated byabsorbing heat from the previously heated heat-sink material, therebycooling the heat-sink media. After a second period of time, the flow ofcold gas to the regenerative heat exchanger module is shutoff and thehot gas is again re-introduced into the regenerative heat exchangermodule. The above cycle is repeated as long as required. The periods forthe flow of hot and cold gas through each of the regenerative heatexchanger module are staggered so that the hot and cold gasesprogressively flow through each of the stationary regenerative heatexchanger modules, thereby simulating the operation of a rotaryregenerative heat exchanger having revolving heat transfer sectors.

The dimensions of the regenerative heat exchanger modules are selectedto facilitate factory assembly, testing, and shipment of the module byroad or rail and for easy installation of the module at the job-site.Inlet and outlet dampers on the hot and cold gas sides of theregenerative heat exchanger module control the flow of the hot and coldgases. The damper blades are moved by pneumatic, hydraulic or electricactuators.

The heat sink media in the regenerative heat exchanger module can bemetallic or refractory plates, which can further be configured asmulti-layered monolith blocks. The flow of the hot and cold gas througheach regenerative heat exchanger module can be in the same or inopposite directions to each other. Each of the regenerative heatexchanger modules can process either an equal fraction or an unequalfraction of the total flow quantity of hot or cold gas through theregenerative heat exchanger system. Yet further, the number ofregenerative heat exchanger modules receiving the hot gas can either beequal or not equal to the number of regenerative heat exchanger modulesthat receive the cold gas.

Each of the regenerative heat exchanger modules can have an idle mode ofoperation wherein the flow of the hot and cold gases to the regenerativeheat exchanger module is temporarily shut off while the regenerativeheat exchanger module is in transition between the hot and cold gases.Yet further, each of the regenerative heat exchanger modules can betaken off-line for repairs or maintenance by manual selection throughthe regenerative heat exchanger system controller. Alternately, theregenerative heat exchanger module can be automatically taken off-lineby the regenerative heat exchanger system controller whenever theflowrate of the gases drops as a result of reduced demand on the boiler.While the regenerative heat exchanger module is off-line, the remainingregenerative heat exchanger modules continue to operate to simulate arotary regenerative heat exchanger.

Another aspect of the invention includes a regenerative heat exchangersystem controller. The regenerative heat exchanger system controller canbe an electronic programmable computer, which executes computer code foroperating each regenerative heat exchanger module and for staggering theoperation of each regenerative heat exchanger module to simulate theoperation of a rotary regenerative heat exchanger. The regenerative beatexchanger system controller can include an idle mode of operation andmeans for manually or automatically selecting a regenerative heatexchanger module for off-line operation as described above.

Yet another aspect of the invention includes a method of operating anumber of independently operable regenerative heat exchanger modules tosimulate the operation of a rotary regenerative heat exchanger. Themethod includes passing a first gas through a regenerative heatexchanger module for a first period of time and then optionally idlingit for a brief period of time and then passing a second gas through theregenerative heat exchanger module for a second period of time. Theoperation of each of the regenerative heat exchanger modules isstaggered to simulate the operation of a rotary regenerative heatexchanger. Upon the shutdown of a regenerative heat exchanger module,the remaining regenerative heat exchanger modules are continued tooperate to simulate the operation of a rotary regenerative heatexchanger. Further, the period of operation with the first and secondgases can be kept the same or can be changed when the remainingregenerative heat exchanger modules are operated.

These and other features, aspects, and advantages of the presentinvention will become better understood with reference to the followingdrawings, description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C are isometric representations, which show thedetails of different embodiments of a regenerative heat exchanger moduleaccording to the present invention.

FIG. 2 is an isometric representation of a regenerative heat exchangersystem, which is assembled from a plurality of the regenerative heatexchanger modules shown in FIG. 1A.

FIG. 3 is a schematic representation of an electrical control diagram,which shows the control systems for the regenerative heat exchangersystem of FIG. 2 and the regenerative heat exchanger modules of FIG. 1A.

FIG. 4 is a flow-chart which shows the control logic for the controlsystem for the regenerative heat exchanger system of FIG. 2.

FIG. 5 is a flow-chart which shows the control logic for the controlsystem for the regenerative heat exchanger module of FIG. 1A.

FIG. 6 shows Tables 3,4,5,6,7, and 8 which represent the differentoperating modes of the regenerative heat exchanger modules in aregenerative heat exchanger system.

DETAILED DESCRIPTION

Referring now to FIGS. 1A, and 2, FIG. 1A shows a partially cut-awayisometric representation of a single regenerative heat exchanger moduleaccording to the present invention and FIG. 2 shows a plurality ofregenerative heat exchanger modules of FIG. 1A assembled intoregenerative heat exchanger system 100.

As shown in FIG. 2, regenerative heat exchanger system 100 is comprisedof a number of regenerative heat exchanger modules. FIG. 2, shows as anexample, five regenerative heat exchanger modules, which are representedby the reference numerals 1, 2, 3, 4, and 5. The five regenerative heatexchanger modules could be identical to each other or they could havesome variations depending on design requirements.

FIG. 1A shows details of a typical regenerative heat exchanger module,in this case regenerative heat exchanger module 1. Regenerative heatexchanger module 1 is shown configured as an H-shaped housing 1 h havinga middle section 1.1 and ducts 1.3, 1.4, 1.5, and 1.6. Housing 1 h canbe fabricated of carbon-steel or stainless steel or other suitablematerial and can be internally or externally insulated to reduce heatlosses. As seen in the cutaway representation of regenerative heatexchanger module 1 of FIG. 1A, the middle section 1.1 is configured as abox with an open top and bottom and a right side 1.1 r, a left side 1.1l, a front side 1.1 f, and a back side 1.1 b.

A heat sink media 1.2 is contained within the middle section 1.1. Heatsink media 1.2 is supported on heat sink media support 1.7, which islocated near the lower end of middle section 1.1. Heat sink media 1.2can be any standard heat sink media, which is suitable for use inconventional regenerative heat exchangers. For example, it could bestructured metallic plates as described in U.S. Pat. No. 5,318,102 toSpokoyny et al. Alternatively, it could be made of a refractory materialsuch as ceramic stoneware or porcelain. As an example, it could beceramic structured media blocks such as those available from LantecProducts Inc, USA under the trade-name of Multi-Layered Monolith Media(MLM®) and described in U.S. Pat. No. 5,852,636 to Lang et al.Alternatively, it could be random packing such as ceramic saddles, alsoavailable from Lantec Products Inc, USA or metal rods or balls or othershapes that could be poured to form a packed heat-storage bed. All theseforms of heat-sink material are well known in the art.

The upper free volume in middle section 1.1 above heat sink media 1.2 isshown by the reference numeral 1.1 x in FIG. 1A.

Heat sink media support 1.7 is any means of supporting heat sink media1.2 such as grates or perforated plates which are capable of supportingthe weight of heat sink media 1.2 while allowing the hot or cold gas toflow into heat sink media 1.2. Such means of support are well describedin the prior art such as U.S. Pat. No. 5,770,165 to Truppi et al.

The lower free volume in middle section 1.1 under the heat sink mediasupport 1.7 is shown by the reference numeral 1.1 y in FIG. 1A.

The vertical arms and legs of housing 1 h are configured as ducts 1.3,1.4, 1.5, and 1.6 respectively for the flow of the hot and cold gasesinto and out of housing 1 h. While the ducts are shown as square orrectangular cross-sectioned in FIGS. 1A, 1B, 1C, and 2, they could alsobe circular or any other shape in cross-section. Duct 1.3 has an openend 1.3.1, which allows for the flow of hot gas H into housing 1 h.Similarly, duct 1.6 has an open end 1.6.1, which allows for the flow ofthe cooled hot gas shown as H′ out of housing 1 h.

A flow control means is located in duct 1.3 to control the flow of thehot gas H into housing 1 h. In FIG. 1A, flow control means is shown as abutterfly damper 1.3.2. However, the flow control means could also beany other valve device such as a poppet valve (as shown in FIG. 1B), aguillotine damper, a two-way diverter damper (as shown in FIG. 1C), orany other device which controls the flow of a gas. Such flow controldevices are well-known in the art and are available from USmanufacturers such as Precision Engineered Products Inc., Mosser DampersInc., Bachmann Dampers Inc. and others.

Similarly, a butterfly damper 1.6.2 is shown located in duct 1.6 forcontrolling the flow of cooled hot gas H′ out of housing 1 h.

During operation of regenerative heat exchanger module 1, hot gas Henters inlet 1.3.1 into duct 1.3 and flows past open damper 1.3.2 intoupper volume 1.1 x of middle section 1.1. The hot gas H then flowsdownwards through heat sink media 1.2. Since heat sink media 1.2 isrelatively cooler than hot gas H as a result of a previous flow of coldgas C through it (as will be described below), hot gas H gives up itsheat to heat transfer media 1.2. Therefore heat sink media 1.2 is heatedwhile hot gas H is cooled to cooled hot gas H′. Cooled hot gas H′ thenflows downwards through media support 1.7 into lower volume 1.1 y ofmiddle section 1.1. The cooled hot gas H′ then flows into duct 1.6 andflows past open damper 1.6.2 to outlet 1.6.1 from where it exits housing1 h.

Similarly, duct 1.5 of housing 1 h has an open end 1.5.1, which allowsfor the flow of cold gas C into housing 1 h. Also, duct 1.4 has an openend 1.4.1, which allows for the flow of heated cold gas shown as C′ outof housing 1 h. Damper 1.5.2 is located in duct 1.5 to control the flowof cold gas C into housing 1 h. Similarly, damper 1.4.2 is located induct 1.4 for controlling the flow of heated cold gas C′ out of housing 1h. During operation of regenerative heat exchanger module 1, the coldgas C enters inlet 1.5.1 into duct 1.5 and flows past open damper 1.5.2into lower section 1.1 y of middle section 1.1. Cold gas C then flowsupwards through heat sink media support 1.7 into heat sink media 1.2.

Since, as described above, heat sink media 1.2 has been previouslyheated by the flow of hot gas H, heat sink media 1.2 is at a relativelyhigher temperature than cold gas C. Heat sink media 1.2 therefore givesup its heat to cold gas C which in turn is heated to heated cold gas C′.Heated cold gas C′ then flows out of the cooled heat sink media 1.2 intoupper volume 1.1 x of middle section 1.1. The heated cold gas then lowsinto duct 1.4 and flows past open damper 1.4.2 to outlet 1.4.1 fromwhere it exits housing 1 h.

Dampers 1.3.2, 1.4.2, 1.5.2, and 1.6.2 can all be of the same type suchas butterfly dampers or they can be of different types such as two-waydiverter dampers (as shown in FIG. 1C) or poppet dampers (as shown inFIG. 1B) without departing from the spirit of the invention. Further, asshown in FIG. 3, individual actuators 1.3 a, 1.4 a, 1.5 a, 1.6 a canoperate each of dampers 1.3.2, 1.4.2, 1.5.2, and 1.6.2. Alternately, oneor more common actuators can be used in any combination to operatedampers 1.3.2, 1.4.2, 1.5.2, and 1.6.2. As is well known, actuators 1.3a, 1.4 a, 1.5 a, 1.6 a can be operated by electrical energy or by apressurized fluid such as compressed air or hydraulic fluid. The use ofactuators to operate dampers in the regenerative heat exchanger modulesis well known in the art. For example, pneumatically or hydraulicallyactuators (commercially available from Parker-Hannifin Inc., USA orother manufacturers) or electrically operated actuators (commerciallyavailable from Foxboro-Jordan Inc, USA or other manufacturers) could beused to move the dampers. As will be described below, these actuatorscan be controlled according to a pre-programmed sequence of operation byelectrical control logic such as relays or by computer control meanssuch as micro-controllers or Programmable Logic Controllers.

As is well known in the regenerative heat-exchanger art, dampers 1.3.2,1.6.2, 1.5.2, and 1.4.2 are operated so that only the hot gas or thecold gas can flow through housing 1 h at any given time. Dampers 1.3.2,1.6.2, 1.5.2, and 1.4.2 of regenerative heat exchanger module 1 arecontrolled by a regenerative heat exchanger module controller 1 p, whichis described in detail in the descriptions of FIGS. 3 and 5. Initially,regenerative heat exchanger module controller 1 p opens dampers 1.3.2and 1.6.2 and closes dampers 1.5.2 and 1.4.2 to enable hot gas H only toflow through the previously cooled heat sink media 1.2. After a firstperiod of time, after heat sink media 1.2 has been heated to a requiredlevel or hot gas H has been cooled to a required maximum level,regenerative heat exchanger module controller 1 p closes dampers 1.3.2and 1.6.2 to shut off the flow of hot gas H into housing 1 h.Regenerative heat exchanger module controller 1 p then opens dampers1.5.2 and 1.4.2 to allow the flow of cold gas C into housing 1 h. Aftera second period of time, after heat sink media 1.2 has been cooled to arequired level or cold gas C has been heated to a required level,regenerative heat exchanger module controller 1 p closes dampers 1.5.2and 1.4.2 to shut off the flow of cold gas C into housing 1 h. Thesecond period of time may or may not be equal to the first period oftime. Regenerative heat exchanger module controller 1 p then opensdampers 1.3.2 and 1.6.2 to allow the flow of hot gas H into housing 1 hto heat again the cooled heat sink media 1.2. Regenerative heatexchanger module controller 1 p repeats this cycle of heating andcooling heat sink media 1.2 by alternately flowing hot gas H and coldgas C through it as long as required for the operation of regenerativeheat exchanger module 1.

As shown in FIG. 3, regenerative heat exchanger module controller 2 p, 3p, 4 p, and 5 p are provided for each of the other regenerative heatexchanger modules 2, 3, 4, and 5 respectively. Regenerative heatexchanger module controller 2 p, 3 p, 4 p, and 5 p control the operationof each of the actuators of regenerative heat exchanger modules 2, 3, 4,and 5 respectively. Thereby, a sequence of actuator operations, similarto that described above for regenerative heat exchanger module 1, canalso be performed for each regenerative heat exchanger modules 2, 3, 4,and 5 in FIG. 2. Thus each of regenerative heat exchanger modules 1, 2,3, 4, and 5 is capable of operating as individual regenerative heatexchangers for transferring heat from a hot gas H to a cold gas C. Aswill be described below with reference to FIG. 2, the individuallyoperable regenerative heat exchanger modules 1, 2, 3, 4, and 5 arecombined into regenerative heat exchanger system 100 for simulating theoperation of a rotary regenerative heat exchanger.

While the physical configuration of regenerative heat exchanger module 1has been shown to be H-shaped as described above, other configurationscan also be used without departing from the spirit of the invention.

As shown in FIG. 2, regenerative heat exchanger modules 2, 3, 4, and 5are constructed and operated similar to regenerative heat exchangermodule 1. Thus regenerative heat exchanger module 2 has heat sink media2.2, hot gas inlet damper 2.3.2, cooled hot gas outlet damper 2.6.2,cold gas inlet damper 2.5.2, and heated cold gas outlet damper 2.4.2;regenerative heat exchanger module 3 has heat sink media 3.2, hot gasinlet damper 3.3.2, cooled hot gas outlet damper 3.6.2, cold gas inletdamper 3.5.2, and heated cold gas outlet damper 3.4.2; regenerative heatexchanger module 4 has heat sink media 4.2, hot gas inlet damper 4.3.2,cooled hot gas outlet damper 4.6.2, cold gas inlet damper 4.5.2, andheated cold gas outlet damper 4.4.2; and regenerative heat exchangermodule 5 has heat sink media 5.2, hot gas inlet damper 5.3.2, cooled hotgas outlet damper 5.6.2, cold gas inlet damper 5.5.2, and heated coldgas outlet damper 5.4.2.

It is not necessary that regenerative heat exchanger modules 2, 3, 4,and 5 be identical to regenerative heat exchanger module 1 as long theypossess the major features described above for regenerative heatexchanger module 1. As shown in FIG. 2, common duct manifolds 8 c, 8 c′,8 h, and 8 h′ are provided to individually route the cold gas C, theheated cold gas C′, the hot gas H′, and the cooled hot gas H′ to andfrom the regenerative heat exchanger system 100. Each of the ductmanifolds have individual openings that are connected to each of theopenings (for example, openings 1.3.1, 1.4.1, 1.5.1, and 1.6.1 ofregenerative heat exchanger module 1) in each of the regenerative heatexchanger modules 1, 2, 3, 4, and 5 of regenerative heat exchangersystem 100.

In FIG. 2, five regenerative heat exchanger modules have been shown tocreate regenerative heat exchanger system 100. However, it will beobvious that any number of regenerative heat exchanger modules could beused to create regenerative heat exchanger system 100. The number ofregenerative heat exchanger modules will depend on the individual flowcapacity of each of the regenerative heat exchanger modules and thetotal flow requirements of regenerative heat exchanger system 100. Thusin the case of the regenerative heat exchanger system 100 shown in FIG.2, each of the regenerative heat exchanger modules would processone-half of the total flow of the hot gas or the cold gas respectively.As another example, each module in a regenerative heat exchanger systemcontaining “N” modules would treat “2/N” of the total flow of the hot orcold gases that are flowed through the system if “N” is an even integer.Alternatively, each module would treat “2/(N-1)” of the total flow ofthe hot and cold gases that are flowed through the regenerative heatexchanger system if “N” is an odd integer assuming one regenerative heatexchanger module is idled. However, it is not necessary that each of theregenerative heat exchanger modules be sized to treat equal amounts ofthe hot or cold gas. They could even be sized to treat differentfractions of the total quantity of hot or cold gas to be processed bythe regenerative heat exchanger system. For example, to accommodateplant space or other requirements, four modules could be designed toprocess 10%, 20%, 30%, and 40% of the total flow of the hot gasrespectively.

It is also not necessary that the regenerative heat exchanger modules beidentical to each other as shown in FIG. 2. For example, the positionsof hot gas and cold gas inlet and outlet ducts can be changed toaccommodate plant duct layout requirements without departing from thespirit of the invention.

Further, shipping dimension constraints would also determine the sizeand number of regenerative heat exchanger modules required to create aregenerative heat exchanger system for any given application. As apractical consideration, the dimensions of the regenerative heatexchanger modules would be less than 180 inches wide by 168 inches highfor shipping by road or rail. This geometry is very advantageous formanufacture of the regenerative heat exchanger modules in a fabricationshop. In such a situation, the regenerative heat exchanger modules couldbe completely assembled with dampers, actuators, controls, and heat-sinkmedia in the fabrication shop. The completely assembled regenerativeheat exchanger modules can then be easily shipped by road or rail to thepower-plant or other installation site. Each of the completely assembledregenerative heat exchanger modules can then be easily dropped intoplace in the desired location at the installation site to provide thecomplete regenerative heat exchanger system shown in FIG. 2.

For economy, ease of manufacturing, and assembly, it is advantageousthat the regenerative heat exchanger modules be identical. This wouldgreatly speed up the manufacture of the regenerative heat exchangermodules and facilitate the assembly of the regenerative heat exchangermodules into the regenerative heat exchanger system as described above.This is a great advantage over current state of the art rotaryregenerative heat exchangers, which are too large to ship in one pieceand therefore require large amounts of fieldwork and downtime forinstallation. The use of the regenerative heat exchanger modules asdescribed in the claimed invention will greatly reduce the overall costand time required for installation of a regenerative heat exchanger.

Yet another advantage of the use of regenerative heat exchanger moduleslies in the use of pressure-sealed dampers to control flow. As describedin the previously referenced US patent to Finnemore, rotary regenerativeheat exchangers have a severe sealing problem which permits thecross-leakage of the hot gas into the cold gas or vice versa. Forexample, it is known that the typical sliding seal in the rotary heatexchanger used in a power-plant boiler will leak almost 20 to 25 percentof the combustion air into the flue gas stream after the seal has beenin operation for a relatively short period of time. At this point, thethermal efficiency of the rotary regenerative heat exchanger dropssharply and operation of the rotary regenerative heat exchanger becomesuneconomical necessitating the replacement of the seal. Further, theleakage causes a parasitic power usage of up to 10% of the electricalpower produced by the power-plant for the sole purpose of raising theleaked, unused combustion air to the required static pressure requiredfor the operation of the boiler. Replacement of the seal requires thatthe power-plant be shut down and results in lost production anduneconomical operation of the power-plant.

In comparison to the above situation, the dampers used in theregenerative heat exchanger modules of the present invention have almostnegligible (expected to be less than 0.5 percent) leakage between thehot and cold gases during its operating lifetime. Further, these dampersdo not have any sliding seals, which require frequent replacement. Thusdown-time is reduced resulting in more economical operation andincreased utilization of the power-plant.

Yet another advantage of using the regenerative heat exchanger modulesdescribed above is that the heat sink material is stationary. Therefore,the complex mechanical drive system, which is used to rotate the wheelcontaining the heat-sink material in the rotary regenerative heatexchanger, is not required.

Regenerative heat exchanger system 100 can be operated to simulate theoperation of a rotary regenerative heat exchanger as shown in Table-1which shows the sequence of operation of regenerative heat exchangermodules 1, 2, 3, 4, and 5. In Table-1, H=Heating bed wherein the hot gasdampers are open to admit hot gas into the heat sink media bed to “heat”the bed while the cold gas dampers are closed to prevent the flow ofcold gas into the bed; C=Cooling bed wherein the cold gas dampers areopen to admit cold gas into the heat sink media bed to “cool” the bedwhile the hot gas dampers are closed to prevent the flow of hot gas intothe bed; I=Idle bed where all dampers are closed to prevent the flow ofeither the hot gas or the cold gas into the heat sink media bed; andT=Transition bed where the hot gas damper is moving from an open orclosed position to a closed or open position or where the cold gasdamper is moving from an open or closed position to a closed or openposition.

In Table-1, the column for each module represents the operation of thecontroller for that regenerative heat exchanger module while the entiretable represents the operation of the controller 100 p for theregenerative heat exchanger system 100. The control logic for eachindividual regenerative heat exchanger module is shown using the controllogic for regenerative heat exchanger module 1 as an example in FIG. 5.The control logic for regenerative heat exchanger system 100 is shown inFIG. 4. It will be obvious that any odd number, greater than or equal tothree, of regenerative heat exchanger modules can be utilized using thesame control philosophy even though Table-1 shows a regenerative heatexchanger system containing 5 regenerative heat exchanger modules. TABLE1 Regenerative heat exchanger Module No: 1 2 3 4 5 Module Controller 1p2p 3p 4p 5p No: Module H H I C C Operating Mode: T H T C C I H H C C T HH T C C H H I C C T H T C C I H H C C T H H T C C H H I C C T H T C C IH H T C T H H I C C H H T C C T H H C C I H H T C T H H I C C H H T C CT H H C C I H H T C T H H I C C (Original condition at start of table)

Referring to Table-1, regenerative heat exchanger system controller 100p instructs regenerative heat exchanger module controllers 1 p and 2 pto open dampers 1.3.2, 1.6.2, 2.3.2, and 2.6.2 and close dampers 1.4.2,1.5.2, 2.4.2, and 2.5.2 in regenerative heat exchanger modules 1 and 2respectively. Thus, regenerative heat exchanger modules 1 and 2 areinitially in a “heating” mode wherein the hot gas H is flowed throughheat sink media 1.2 and 2.2.

Similarly, regenerative heat exchanger system controller 100 p instructsregenerative heat exchanger module controllers 4 p and 5 p to opendampers 4.4.2, 4.5.2, 5.4.2, and 5.5.2 and close dampers 4.3.2, 4.6.2,5.3.2, and 5.6.2 in regenerative heat exchanger modules 4 and 5respectively. Thus regenerative heat exchanger modules 4 and 5 are in a“cooling” mode wherein the cold gas C is flowed through heat sink media4.2 and 5.2. In the control system shown in Table-1, the heating modeperiod is equal to the cooling mode period. However, other controlschemes with unequal heating mode and cooling mode periods can also beused.

Initially, in Table-1, regenerative heat exchanger system controller 100p instructs regenerative heat exchanger module controller 3 p to closeall four dampers. Thus regenerative heat exchanger module 3 is an “idle”mode wherein neither the hot gas H nor the cold gas C is flowed throughit. The idle mode time is equal to P/M where P=heating modeperiod=cooling mode period (including damper transition times) andM=number of regenerative heat exchanger modules receiving the hot gaswhen the subject regenerative heat exchanger module is idle. Thus, inTable-1, if P equals 20 seconds, then M would equal 2 and the idle modetime would equal 10 seconds.

Controller 100 p for operation of regenerative heat exchanger system 100as represented by Table-1 is designed so the flow of hot gas H can beenabled in regenerative heat exchanger module 1 before it is enabled inregenerative heat exchanger module 2. Therefore, in Table-1,regenerative heat exchanger module 1 is more advanced in the heatingmode than regenerative heat exchanger module 2. Similarly, thecontroller for operation of the regenerative heat exchanger systemrepresented by Table-1 is designed so that the flow of cold gas C wasenabled in regenerative heat exchanger module 4 before it was enabled inregenerative heat exchanger module 5. Therefore, in Table-1,regenerative heat exchanger module 4 is more advanced in the coolingmode than regenerative heat exchanger module 5.

When regenerative heat exchanger module 1 has reached the end of itsheating mode period, regenerative heat exchanger system controller 100 pinstructs regenerative heat exchanger module controller 1 p to closedampers 1.3.2 and 1.6.2 of regenerative heat exchanger module 1.Simultaneously, regenerative heat exchanger system controller 100 p alsoinstructs regenerative heat exchanger module controller 3 p to opendampers 3.3.2 and 3.6.2 of previously idle regenerative heat exchangermodule 3. During the opening and closing of the dampers, regenerativeheat exchanger modules 1 and 3 are in a transition mode (represented by“T” in Table-1).

It is not necessary that the regenerative heat exchanger systemcontroller 100 p and the regenerative heat exchanger module controllers1 p, 2 p, 3 p, 4 p, and 5 p be located in separate physical instruments.For example, a single programmable logic controller could be used as theregenerative heat exchanger system controller and the regenerative heatexchanger module controllers wherein the functions of the variouscontrollers are located in different parts of a software program. Thus,the regenerative heat exchanger system controller 100 p could be writtenas a parent program, which invokes child programs or sub-programs orsub-routines that in turn function as regenerative heat exchanger modulecontrollers 1 p, 2 p, 3 p, 4 p, and 5 p.

The stroke times of the actuators in the regenerative heat exchangermodules are adjusted so the time required to open the dampers is equalto the time required to close the dampers. Thus, in Table-1, whendampers 1.3.2 and 1.6.2 are fully closed, dampers 3.3.2 and 3.6.2 arefully opened. Therefore, there is no flow of gas through regenerativeheat exchanger module 1, which now becomes an idle regenerative heatexchanger module. The hot gas now flows through regenerative heatexchanger module 3, which is now in the heating mode.

After the idle period has elapsed, regenerative heat exchangercontroller 100 p conducts a similar sequence of operations to convertregenerative heat exchanger module 4 from a cooling mode to an idle modeand regenerative heat exchanger module 1 from an idle mode to a coolingmode. As described above, regenerative heat exchanger system controller100 p instructs regenerative heat exchanger module controllers 4 p and 1p to close dampers 4.4.2 and 4.5.2 in regenerative heat exchanger module4 and open dampers 1.4.2 and 1.5.2 in regenerative heat exchanger module1.

The above sequences of operations are carried out for all theregenerative heat exchanger modules so that each regenerative heatexchanger module experiences an idle mode, a heating mode, an idle mode,and a cooling mode. This operation is similar to the operation of arotary regenerative heat exchanger. The regenerative heat exchangersystem of the present invention therefore simulates the operation of therotary regenerative heat exchanger without the above-described inherentdisadvantages of the rotary regenerative heat exchanger.

While Table-1 shows the damper sequence for a regenerative heatexchanger system having an odd number, greater than or equal to, threeof regenerative heat exchanger modules, a similar damper sequence canalso be developed for a regenerative heat exchanger system having aneven number, greater than or equal to two, of regenerative heatexchanger modules. However, in such regenerative heat exchanger systems,there is no pre-set idle mode of operation. As an example, Table-2 showsthe damper sequence for a regenerative heat exchanger system having fourregenerative heat exchanger modules. TABLE 2 Regenerative heat exchangerModule No: 1 2 3 4 Operating Mode: H H C C T H T C C H H C C T H T C C HH T C T H H C C H H T C T H H C C (Original condition at start of table)Yet other operating schemes can be devised according to the controlphilosophy described above. For example, the five regenerative heatexchanger modules shown in FIG. 2 can be operated according to thedamper control sequence of Table-1 to provide the maximum thermalefficiency with the minimum pressure fluctuation or cross-leakage of thehot and cold gas streams. In the event of a disabling problem with oneof the regenerative heat exchanger modules, the other four regenerativeheat exchanger modules can be temporarily operated according to thedamper control sequence of Table-2 because each of the regenerative heatexchanger modules is independently operable. This procedure willmaintain the operating thermal efficiency with a temporary increase incross-leakage and pressure fluctuations. This is a major advantage overthe rotary regenerative heat exchanger, which has to be totally shutdown in case of a problem within any of its heat-transfer sectors.

Similarly, individual regenerative heat exchanger modules can be takenoff-line during the night or at other times when reduced electricaldemand on the power plant requires that the boiler be operated atreduced capacity. This is a major advantage over the rotary regenerativeheat exchanger of the present art wherein such operation generallyresults in reduced flows and velocities of the hot and cold gasesthrough the rotary regenerative heat exchanger system. Under suchoperating conditions, the deposition of flyash on the heat sink materialsurfaces of the rotary regenerative heat exchanger is greatly increasedrequiring more frequent soot-blowing and increasing the operating costof the rotary regenerative heat exchanger.

In contrast to the rotary regenerative heat exchanger, the modularregenerative heat exchanger system of the present invention can bedesigned and operated with varying numbers of regenerative heatexchanger modules to maintain a high design velocity even when the gasflowrate is reduced.

For example, a regenerative heat exchanger system can be designed andoperated with seven regenerative heat exchanger modules to accommodate100 percent of the flowrate of the flue-gas, which is generated by theboiler during the daytime. At nighttime, when the boiler is operated atreduced capacity due to reduced electrical demand on the power plant,the regenerative heat exchanger system can be operated with five orthree regenerative heat exchanger modules, as required to accommodatethe reduced flowrate of the flue-gas. Operation of the reduced number ofregenerative heat exchanger modules at the design flowrate maintainshigh gas velocities within the heat sink material. The high gasvelocities in turn minimize the potential for particulate matter such asflyash in the flue gas stream to deposit within and plug up the flowpassages in the heat sink material. Less frequent cleaning of the heatsink material surfaces is required which results in reduced steam orcompressed air consumption for soot-blowing, reduced wear and tear ofthe heat sink material due to the operation of the soot-blower, andreduced operating and maintenance costs.

The number of regenerative heat exchanger modules can be manually orautomatically controlled in response to the operating capacity of theboiler. As shown in FIG. 3, a boiler capacity sensing means 100 s can beused to provide a signal to regenerative heat exchanger systemcontroller 100 p to modulate the number of regenerative heat exchangermodules in operation in accordance with the control flow-logic shown inFIG. 4. Boiler capacity sensing means 100 s can measure an operatingvariable such as coal fuel or liquid fuel or natural gas fuel firingrate or combustion air flowrate or flue-gas flowrate or boiler feedwater flowrate or megawatt-load demand of the electrical grid system orany other operating parameter which can be correlated to the boileroperating level. Such systems are well known in the boiler industry.

Boiler capacity sensing means 100 s then provides a signal to theregenerative heat exchanger system controller 100 p to indicate theboiler operating level. Regenerative heat exchanger system controller100 p then determines the number of regenerative heat exchanger modulesrequired for optimum operation based upon pre-programmed algorithmswhich correlate the number of modules to the boiler operating level.Thus, regenerative heat exchanger system controller 100 p automaticallymodulates the number of operating regenerative heat exchanger modules toprovide optimum flow velocities in the heat sink material to maximizeheat transfer efficiency and reduce flyash deposition.

Another advantage of the system is that selected individual regenerativeheat exchanger modules can be isolated from the system for maintenanceas required while the boiler is in operation without shutting down theboiler or the entire regenerative heat exchanger system. Because a largenumber of regenerative heat exchanger modules are used, the removal ofindividual modules from operation does not greatly reduce the boileroperating capacity. For example, in an eleven-module regenerative heatexchanger system, one module can be removed from service therebyreducing operating capacity of the boiler by about 20 percent only ofthe maximum capacity. In comparison, two rotary regenerative heatexchangers are generally used in a conventional boiler system.Therefore, the conventional boiler system capacity is reduced by about50 percent when one of the rotary regenerative heat exchangers isisolated for repairs or service. Thus a boiler equipped with the modularregenerative heat exchanger system as described herein will provide ahigher average annual operating capacity compared to a conventionalboiler which is equipped with rotary regenerative heat exchangers.

The regenerative heat exchanger system described above can also beoperated with unequal flows or unequal heating and cooling periods orboth, As shown as an example in Table 3 in FIG. 6, an eight moduleregenerative heat exchanger system could be operated so that, at anytime, four of the eight modules are receiving the hot gas, three modulesare receiving the cold gas, and the remaining module is idle. As can beseen, the regenerative heat exchanger modules can be operated such thatthe heating period is larger than the cooling period.

Alternatively, as shown in Table 4 of FIG. 6, the eight moduleregenerative heat exchanger can be operated so that, at any time, threeof the eight modules are receiving the hot gas, three modules arereceiving the cold gas, and the remaining two modules are idle. In thiscase, the regenerative heat exchanger modules can be operated such thatthe heating period is equal to the cooling period.

As shown in Tables 5, 6, 7, and 8 of FIG. 6, any combination of heating,cooling, and idle modules can be chosen to simulate the operation of arotary regenerative heat exchanger with a variable number of heattransfer sectors. Also, the heating and cooling periods can be kept thesame as in the original operation or they can be changed to keep theoriginal total cycle time. Thus, as an example in Table 5, oneregenerative heat exchanger module is taken off-line (as indicated by“O”), three regenerative heat exchanger modules are heating, threeregenerative heat exchanger modules are cooling, and one regenerativeheat exchanger module is idle. As a further example, in Table 6, fourregenerative heat exchanger modules are heating, three regenerative heatexchanger modules are cooling, and one regenerative heat exchangermodule is off-line. In Tables 5 and 6, the new cycle time, with thereduced number of regenerative heat exchanger modules in operation, canbe shortened to 70 seconds by maintaining the original switch time of 10seconds. Alternately, the cycle time can be maintained at 80 seconds,even with the reduced number of regenerative heat exchanger modules inoperation, by increasing the switch time to 8/7*10 or 11.43 seconds.

As yet another example, in Table 7, three regenerative heat exchangermodules are heating, three regenerative heat exchanger modules arecooling, and two regenerative heat exchanger modules are off-line. As afinal example, in Table 8, two regenerative heat exchanger modules areheating, three regenerative heat exchanger modules are cooling, andthree regenerative heat exchanger modules are off-line. In Tables 7 and8, the new cycle time, with the reduced number of regenerative heatexchanger modules in operation, can be shortened to 60 seconds bymaintaining the original switch time of 10 seconds. Alternately, thecycle time can be maintained at 80 seconds, even with the reduced numberof regenerative heat exchanger modules in operation, by increasing theswitch time to 8/6*10 or 13.33 seconds. As a general case, theregenerative heat exchanger module system can be maintained at theoriginal cycle time by increasing the switch time to (N/N-O))*S where Nequals the total number of originally operating modules, O equals thenumber of off-line modules, and S equals the original switch time.

From the above examples, it will be obvious, that any combinations ofheating, cooling, idle, and off-line regenerative heat exchanger modulescan be operated at any time. This feature is especially usefully, forconducting preventative maintenance or repairs to selected regenerativeheat exchanger modules without shutting down the boiler or theregenerative heat exchanger system. Further, this feature is also usefulfor operating the regenerative heat exchanger system under low-loadconditions as dictated by power grid demands on the powerplant. Thisfeature of operating a variable number of regenerative heat exchangermodules provides great operating advantages over rotary regenerativeheat exchanger systems in which the number of regenerative heatexchanger sectors is fixed at design. The invention allows a varyingnumber of regenerative heat exchanger modules to be automatically puton-line or taken off-line in response to boiler operating level withoutaffecting the overall operation of the boiler or the regenerative heatexchanger system.

Yet other advantages of the modular regenerative heat exchanger systemdescribed herein will be obvious to persons skilled in the art. Itshould be understood, of course, that the foregoing relates to preferredembodiments of the invention and that modifications may be made withoutdeparting from the spirit and scope of the invention as set forth in thefollowing claims.

1) A regenerative heat-exchanger system for recovering waste heat from ahot gas by regeneratively transferring the heat from the hot gas to acold gas, the regenerative heat exchanger system comprising: a pluralityof independently operable regenerative heat exchanger modules, eachregenerative heat exchanger module sized for the processing of afraction of the total design quantities of hot and cold gases, eachregenerative heat exchanger module comprising heat-sink material placedin the paths of flow of the hot and cold gases through the regenerativeheat exchanger module, and a flow control means having open and closedpositions located in the paths of flow of the hot and cold gases forcontrolling the flow of the hot and cold gases through the heat-sinkmaterial, and a regenerative heat exchanger module control meansoperably connected to the regenerative heat exchanger module to operatethe flow control means of the regenerative heat exchanger module toenable the alternate flow of the hot and the cold gases through theheat-sink material to provide regenerative heat transfer of the heat inthe hot gas to the cold gas; and a regenerative heat exchanger systemcontrol means operably connected to each regenerative heat exchangermodule control means to sequence the operation of the regenerative heatexchanger module control means of the regenerative heat exchangermodules so that the flow of the hot and the cold gas progressessequentially through each of the regenerative heat exchanger moduleswhereby the operation of a rotary regenerative heat exchanger issimulated. 2) The regenerative heat-exchanger system of claim 1, whereinthe regenerative heat exchanger system control means can select anynumber of regenerative heat exchanger modules from the total number ofregenerative heat exchanger modules present in the regenerative heatexchanger system for operation. 3) The regenerative heat-exchangersystem of claim 1, wherein the dimension of the regenerative heatexchanger module is less than 180 inches. 4) The regenerativeheat-exchanger system of claim 1, wherein the flow control meanscomprises at least one gas flow control damper, the damper having a flowrestricting member which is movable between an open and restrictedposition to enable or restrict the flow of the gas through the damper.5) The regenerative heat-exchanger system of claim 4, further comprisinga pressurized fluid actuator, the actuator connected to the flowrestricting member to effect the movement of the flow restricting memberbetween its open and restricted positions. 6) The regenerativeheat-exchanger system of claim 4, further comprising an electricactuator, the actuator connected to the flow restricting member toeffect the movement of the flow restricting member between its open andrestricted positions. 7) The regenerative heat-exchanger system of claim1, wherein the heat-sink material comprises a refractory material. 8)The regenerative heat-exchanger system of claim 1, wherein the heat-sinkmaterial comprises a metallic material. 9) The regenerativeheat-exchanger system of claim 7, wherein the refractory materialcomprises a ceramic material. 10) The regenerative heat-exchanger systemof claim 1, wherein the flow of the hot gas within the heat sinkmaterial is generally counter-current to the flow of the cold gas. 11)The regenerative heat exchanger system of claim 1, wherein eachregenerative heat exchanger module processes an equal fraction of thetotal design quantity of hot gas. 12) The regenerative heat exchangersystem of claim 1, wherein each regenerative heat exchanger moduleprocesses an unequal fraction of the total design quantity of hot gas.13) The regenerative heat exchanger system of claim 1, wherein eachregenerative heat exchanger module processes an equal fraction of thetotal design quantity of cold gas. 14) The regenerative heat exchangersystem of claim 1, wherein each regenerative heat exchanger moduleprocesses an unequal fraction of the total design quantity of cold gas.15) The regenerative heat exchanger system of claim 1, wherein thefraction of the total design quantity of the hot gas processed in theregenerative heat exchanger module is equal to the fraction of the totaldesign quantity of the cold gas processed in the regenerative heatexchanger module. 16) The regenerative heat exchanger system of claim 1,wherein the regenerative heat exchanger modules are rectangular incross-section, perpendicular to the direction of the flow of the hot andcold gases. 17) The regenerative heat exchanger system of claim 1,wherein equal numbers of regenerative heat exchanger modules arereceiving the hot and cold gases respectively. 18) The regenerative heatexchanger system of claim 1, wherein unequal numbers of regenerativeheat exchanger modules are receiving the hot and cold gasesrespectively. 19) The regenerative heat exchanger system of claim 1,wherein the regenerative heat exchanger module control means comprisesan idle mode of operation wherein the flow control means is temporarilyclosed to generally shut off the flow of hot and cold gases through theregenerative heat exchanger module between the flows of the hot and coldgases into the regenerative heat exchanger module. 20) The regenerativeheat exchanger system of claim 1, wherein the regenerative heatexchanger module control means comprises an off-line mode of operationwherein the flow control means is closed for an extended period of timeto generally isolate a selected regenerative heat exchanger module fromthe hot and cold gases. 21) The regenerative heat exchanger system ofclaim 20, wherein the selected regenerative heat exchanger module isoperated in the off-line mode of operation when the actual quantities ofthe cold and hot gases to the regenerative heat exchanger system areless than the total design quantities of the hot and cold gases. 22) Theregenerative heat exchanger system of claim 20, wherein when theselected regenerative heat exchanger module is operated in the off-linemode, the remaining regenerative heat exchanger modules are continued tooperate in sequence to continue to simulate the operation of the rotaryregenerative heat exchanger. 23) The regenerative heat exchanger systemof claim 1, wherein the regenerative heat exchanger system control meanscomprises an electronic programmable computer. 24) The regenerative heatexchanger system of claim 23, wherein the electronic programmablecomputer executes computer code which controls the operation of eachregenerative heat exchanger module to simulate the operation of a rotaryregenerative heat exchanger. 25) A regenerative heat exchanger systemcontroller for simulating the operation of a rotary regenerative heatexchanger by using a plurality of independently operable regenerativeheat exchanger modules, the control system comprising: a plurality ofregenerative heat exchanger module control means, each regenerative heatexchanger module control means operably connected to an independentlyoperable regenerative heat exchanger module to enable the operation ofthe independently operable regenerative heat exchanger module as aregenerative heat exchanger having at least a heating mode and a coolingmode of operation; and an overall regenerative heat exchanger systemcontrol means operably connected to each regenerative heat exchangermodule control means to control the operation of each of theregenerative heat exchanger module control means so that the heating andcooling modes of operation are sequentially progressed through each ofthe independently operable regenerative heat exchanger modules tosimulate the operation of a rotary regenerative heat exchanger. 26) Theregenerative heat exchanger system controller of claim 25 wherein theoverall regenerative heat exchanger system control means can select anynumber of regenerative heat exchanger modules from the total number ofregenerative heat exchanger modules present in the regenerative heatexchanger system for operation. 27) The regenerative heat exchangersystem controller of claim 25 wherein the control means of each of theregenerative heat exchanger module further comprises an idle mode ofoperation which is also sequentially progressed through each of theregenerative heat exchanger modules to simulate the operation of arotary regenerative heat exchanger having an idle sector. 28) Theregenerative heat exchanger system controller of claim 25 wherein theregenerative heat exchanger module control means further comprises anoff-line mode of operation which generally isolates its associatedregenerative heat exchanger module from the hot and cold gases while theoverall regenerative heat exchanger system control means continues tosequence the operation of the remaining regenerative heat exchangermodules to continue the simulation of the operation of a rotaryregenerative heat exchanger. 29) The regenerative heat exchanger systemcontroller of claim 25 wherein the overall regenerative heat exchangersystem control means comprises an electronic programmable computer. 30)The regenerative heat exchanger system controller of claim 29 whereinthe computer executes computer code which controls the operation of eachof the regenerative heat exchanger module control means to simulate theoperation of a rotary regenerative heat exchanger. 31) A method ofoperating a regenerative heat exchanger system having “M” independentlyoperable regenerative heat exchanger modules, where “M” equals“N1+N2+N3” where “N1”, “N2”, and “N3”, are integers, and “N1” and “N2”define the number of independently operable regenerative heat exchangermodules receiving a first gas and a second gas respectively, the firstand second gases being selected from the set of hot and cold gases, “N3”is the number of idle independently operable regenerative heat exchangermodules equaling 0 or 1, the method comprising the steps of: a) passingthe first gas through a selected regenerative heat exchanger module for“P1” seconds; b) idling the selected regenerative heat exchanger modulefor “P3” seconds where “P3” equals “N3*(P1+P2)/(N1+N2)” wherein “P2” isdefined in step (c); c) passing the second gas through the selectedregenerative heat exchanger module for “P2” seconds; and d) repeatingsteps (a), (b), and (c) for each selected regenerative heat exchangermodule as long as required. 32) The method of claim 31, wherein in step(d), each selected regenerative heat exchanger module is operated insequence and the operation of each selected regenerative heat exchangermodule is staggered with respect to the operation of the otherregenerative heat exchanger modules. 33) The method of claim 32, whereinthe operation of each selected regenerative heat exchanger module isstaggered by “(P1+P2+N3*P3)/(N1+N2+N3)” seconds from the operation ofthe preceding regenerative heat exchanger modules when “N3” equals 1.34) The method of claim 32, wherein the operation of each selectedregenerative heat exchanger module is staggered by“(P1+P2+N3*P3)/(N1+N2+N3)” seconds from the operation of the precedingregenerative heat exchanger modules when “N1” is not equal to “N2”. 35)The method of claim 31, further comprising the steps of e) isolating oneof the regenerative heat exchanger modules from the flow of the firstand second gases; and f) operating the remaining modules in a fallbackmode of operation wherein either “N1” or “N2” or “N3” is reduced by oneto adjust for the non-operational regenerative heat exchanger moduletherein and executing step (d) as long as required. 36) The method ofclaim 35, wherein if the regenerative heat exchanger module isolated instep (e) is in the set of “N1” regenerative heat exchanger modules whichreceive the first gas, “P1” is reduced to “P1*(N1-1)/N1” before step (f)is executed. 37) The method of claim 35, wherein if the regenerativeheat exchanger module isolated in step (e) is in the set of “N1”regenerative heat exchanger modules which receive the first gas, thevalue of “P1” is unchanged while step (f) is executed.